Plasma etching is a process involving the selective removal of material by reactive free radicals or ions generated within a plasma. A plasma is an ionized gas in which concentrations of positive and negative ions are almost equal. The plasma may also contain free radicals which are electrically neutral yet highly reactive. Typically, a plasma is formed by introducing a predetermined gas into a plasma chamber and applying a radio frequency (RF) field to the plasma chamber. The gas introduced is chosen such that it will participate in the intended chemical reaction of a particular process. The RF field causes electron collisions with neutral or charged species to emit radiation. During the etching of semiconductor layer materials, halogen-containing compounds are commonly used in the gas phase as an etching gas.
One of the problems associated with the plasma etching process is that it is generally difficult to control. As a result, the plasma etching process must be continuously monitored to compensate for variations. For example, the etching chamber may be aged, and the etch time required for a freshly cleaned etching chamber will be different from that for an etching chamber that has been in use for some time. Thus, there often exist lot-to-lot variations in wafer attributes and characteristics. This may be minimized by diligent cleaning of the etching chamber. However, the high cleaning cost and the wasteful idled investment productivity dictate that the cleaning (and interruption) should be kept only at a level that is absolutely necessary. In order to balance these two opposite needs, an effective monitoring device is highly needed for the plasma etching process.
At the present time, the etcher chambers are typically monitored by checking the etching rate, particle count, etc. Sometimes, an "open chamber" procedure is utilized. All these procedures are wafer- and time-consuming. The open chamber procedure also causes highly undesirable interruptions in the manufacturing process. Furthermore, the open chamber condition is not the same as when it is in a vacuum.
U.S. Pat. No. 5,653,894 proposes a method which utilizes an active neural network to determine endpoint of a plasma etch process. At least two parameters, such as capacitance, voltage, current, power density, forward and reflected power and other signatures of the RF process are collected and processes using an active neural network to determine endpoint of the plasma etch process. The '894 patent teaches an improved method for analyzing collected parameters, but does not present any new approach as to how these parameters should be collected. The '894 patent also discloses monitoring the wavelength emitted from the wafers using an ellipsometer. However, it also reported that the wavelength used to obtain the ellopsometric traces varied from wafer to wafer and not every trace accurately indicated process endpoint.
U.S. Pat. No. 5,596,207 discloses a monitoring technique for quantifying the effect of plasma/etching during the formation of MOS transistors. The monitor employs a MOS capacitor comprising a thin oxide layer which separates a conductive plate from a semiconductive substrate. The capacitor is modified by placement of a conductive sidewall spacer of the same gate material around the periphery of the conductive plate. Electrical characterization is performed on the resulting modified MOS capacitor to yield information on peripheral damage.
U.S. Pat. No. 5,711,851 discloses a process for improving the performance of a temperature sensitive etch process. In the '851 patent, the temperature of a dry etch process of a semiconductor substrate in a plasma etch chamber is continuously monitored and controlled so as to maintain selectivity and high etch rate. The formation of plasma is terminated when the etching temperature exceeded a predetermined value, and resumed when it falls below another predetermined value.
U.S. Pat. No. 5,643,364 discloses an apparatus for plasma processing of semiconductor wafers or substrates which allows the use of a fixed RF match circuit at high RF frequencies by locating the RF power source at a distance substantially less than an eighth of a wavelength from the coupling device, which can be an electrode or antenna and which couples the RF power into a plasma chamber. The '364 patent also discloses an apparatus for detecting the endpoint of an etch or cleaning process by defecting when the reflected power or VSWR ceases to change as a function of the removal of the substance being etched or cleaned.
The above-mentioned inventions may have their advantages and disadvantages. However, because of the importance of monitoring the plasma etch chamber, especially with respect to the amount of expenses that can be saved and the problems that can be prevented, it is important to explore other, possibly better, monitoring techniques.